1. Field
This disclosure relates generally to a startup procedure for hydrocarbon fuel reformers or fuel processing systems.
2. Description of Related Art
A hydrogen generation subsystem consists of a reformer, hydrogen purification unit and process support components such as valves, sensors, and regulators. Reformers are devices that convert a fuel stream into a hydrogen rich stream. These hydrogen rich streams can also contain gases such as carbon dioxide, carbon monoxide, nitrogen, water vapor, methane, and other gases depending on the composition of the fuel. High temperature gas phase reformers can be of several types including steam reformers, autothermal reformers, and partial oxidation reformers, and hybrids of these reformers. In general, steam reformers use steam to react with the fuel, while autothermal reformers use steam and oxygen, and partial oxidation reformers use only oxygen.
A steam reformer consists of a reform catalyst bed in thermal communication with a combustion source such that heat form the combustor is transferred from the combustion reactions into the reformer catalyst to support the endothermic steam reforming reaction. These systems can be large industrial scale or compact units design for direct integration with a down stream hydrogen use application.
The process of steam reforming has been in industrial practice for over a hundred years and in general several techniques are well known to prevent undesired reactions. These techniques include the following: 1) the addition of excess steam or maintaining the steam to carbon ratio as high as practical based on the thermal efficiency, available reactant supplies, and system complexity that are acceptable; 2) maintaining a hydrogen gas recycle loop within the process; 3) selecting catalyst systems that promote the desired reactions; 4) selecting piping and vessel materials that do not promote the undesired reactions on internal surfaces; 5) operating the reforming process at a steady state condition to maintain a properly designed thermal and pressure profile within the catalyst sections; 6) eliminating or minimizing process transients which can upset the balanced thermal and pressure profiles; 7) establishing a lower temperature pre-reforming section to controllably create a hydrogen rich environment and decrease the average length and/or complexity of the feed compounds prior to the high temperature reform section; and 8) others.
A fuel cell power generator is an example of an application in which compact units are used. In these applications, the fuel cell power generator and therefore the hydrogen generation subsystem and the reformer, may be required to cycle on and off depending on the application and specific load characteristics. This start and stop type operation imposes difficult conditions on the reformer system and can cause unacceptable side reactions. One such side reaction is the formation of solid carbon, which can be due to various reasons including low steam concentrations, cold temperatures, and high pressure prior to reaching stable operating profiles. This is especially true during the startup phase of operation. What is needed is a hydrogen generation system configuration and operating protocol which minimizes the conditions most likely to result in the formation of solid carbon.